Washington: The United States plans to strike the Islamic State group in its Syrian strongholds and could send military advisors into combat alongside Iraqi troops, American commanders said Tuesday.

Military leaders warned of a further escalation in their battle against the jihadists just as two branches of the rival al Qaeda group called for a united front against the war coalition Washington is building.

US warplanes have been targeting IS jihadists in northern Iraq since August 8, and in recent days hit the militants southwest of Baghdad for the first time, in a significant expansion of the campaign.

Meanwhile, Defense Secretary Chuck Hagel told US lawmakers that plans are being laid to hit targets in Syria, where the IS group is holding hostages and has a stronghold in the city of Raqa.

But the US military`s top-ranking officer, General Martin Dempsey, told the same hearing the bombing would not match the huge raids that accompanied the start of the 2003 US-led invasion of Iraq. "This will not look like `shock and awe` because that is not how ISIL is organized, but it will be persistent and sustainable," Dempsey said, using the term Washington used for its 2003 bombardment.

Dempsey also went further that any US official has gone before in admitting that the military advisors that President Barack Obama has dispatched to bolster Iraqi forces could get involved in combat.

Obama`s administration has insisted that his action against the IS extremists is not the start of another US ground war in the Middle East, and that there will be no large-scale American invasion.

But nearly 300 US military advisers are already working with Iraqi government forces, 300 more are on their way and Dempsey refused to rule out their providing "close combat advising."

"To be clear, if we reach the point where I believe our advisors should accompany Iraqi troops on attacks against specific ISIL targets, I will recommend that to the president," Dempsey said.

Dempsey said the advisors are "very much in a combat advisory role" and that there is "no intention" at the moment for them to engage in combat: "I don`t see it to be necessary right now."

But he said if there were an "extraordinarily complex" operation planned by Iraqi forces -- such as a bid to recapture the rebel-held city of Mosul -- then advisors could head to the front.

Dempsey said any use of US troops in the field would be approved by Obama, explaining: "He told me to come back to him on a case-by-case basis."

Obama has vowed to expand American efforts and US diplomats are scrambling to put together an international coalition for a "relentless" campaign against the jihadists.

The slow coming together of this alliance drew a fierce reaction from al Qaeda`s branches in Yemen and in North Africa, who said jihadist forces must also unite against the common threat.

In a joint statement, al Qaeda in the Arabian Peninsula (AQAP) and al Qaeda in the Islamic Maghreb (AQIM) urged their "brothers" in Iraq and Syria to "stop killing each other and unite against the American campaign and its evil coalition that threatens us all."

The Islamic State group began as a successor to al Qaeda`s Iraqi branch, but has escaped from the group`s shadow and clashed with its surrogates in Syria, while claiming leadership of global jihad.

The US strikes against IS fighters in the Sadr al-Yusufiyah area, 25 kilometers (15 miles) from Baghdad, was the first in support of Iraqi forces near the capital.They bring the number of US air strikes across Iraq to 162. The CIA estimates that the Islamic State organization may be able to field

as many as 31,500 fighters -- many of them foreign volunteers.

Iraqi security spokesman Lieutenant General Qassem Atta welcomed the expanded American action, saying the US "carried out an important strike against an enemy target in Sadr al-Yusufiyah."

Sadr al-Yusufiyah lies in the Euphrates Valley, between the militant stronghold of Fallujah and the key battleground of Jurf al-Sakhr, further south. It is one of the closest front lines to Baghdad.

IS militants have seized a swathe of territory in Iraq and Syria, declaring an Islamic "caliphate", committing widespread atrocities and instituting a brutal interpretation of Islamic law.

Western nations and 10 Arab countries, including regional powerhouses Saudi Arabia and Egypt, have agreed to back the US-led campaign, but not all will engage in military action.

Over the weekend, IS militants further upped the stakes in their battle with the West, murdering a British aid worker -- the third Western hostage to be executed on camera.